Quantitative Analysis of Mitochondrial Ca²⁺ Uptake and Release Pathways in Sympathetic Neurons: Reconstruction of the Recovery after Depolarization-Evoked [Ca²⁺]_i Elevations

Rate equations for mitochondrial Ca²⁺ uptake and release and plasma membrane Ca²⁺ transport were determined from the measured fluxes in the preceding study and incorporated into a model of Ca²⁺ dynamics. It was asked if the measured fluxes are sufficient to account for the [Ca²⁺]_i recovery kinetics after depolarization-evoked [Ca²⁺]_i elevations. Ca²⁺ transport across the plasma membrane was described by a parallel extrusion/leak system, while the rates of mitochondrial Ca²⁺ uptake and release were represented using equations like those describing Ca²⁺ transport by isolated mitochondria. Taken together, these rate descriptions account very well for the time course of recovery after [Ca²⁺]_i elevations evoked by weak and strong depolarization and their differential sensitivity to FCCP, CGP 37157, and [Na⁺]_i. The model also leads to three general conclusions about mitochondrial Ca²⁺ transport in intact cells: (1) mitochondria are expected to accumulate Ca²⁺ even in response to stimuli that raise [Ca²⁺]_i only slightly above resting levels; (2) there are two qualitatively different stimulus regimes that parallel the buffering and non-buffering modes of Ca²⁺ transport by isolated mitochondria that have been described previously; (3) the impact of mitochondrial Ca²⁺ transport on intracellular calcium dynamics is strongly influenced by nonmitochondrial Ca²⁺ transport; in particular, the magnitude of the prolonged [Ca²⁺]_i elevation that occurs during the plateau phase of recovery is related to the Ca²⁺ set-point described in studies of isolated mitochondria, but is a property of mitochondrial Ca²⁺ transport in a cellular context. Finally, the model resolves the paradoxical finding that stimulus-induced [Ca²⁺]_i elevations as small as ∼300 nM increase intramitochondrial total Ca²⁺ concentration, but the steady [Ca²⁺]_i elevations evoked by such stimuli are not influenced by FCCP.